Ion source



March 4, 1952 3, w PA -r5 2,587,647

ION SOURCE Filed July 24, 1948 IN VEN TOR. GORDON W. FALL E TTE AT TORNEV Patented Mar. 4, 1952 ION SOURCE Gordon W. Pallette, Pasadena; Calif., assignor to Consolidated Engineering Corporation, Pasadena, Calitl, a corporation of California Application July 24, 1948, Serial N 0. 40,481

11 Claims.

This invention relates in general to mass spectrometers and is directed to improvements in the ion source employed in mass spectrometers and other instruments. More particularly the invention relates to an improved filament and filament mounting means.

A mass spectrometer is an analytical apparatus in which ions may be sorted and measured. Ordinarily, it includes an ion source into which the sample to be analyzed is introduced. In the ion source, the sample is bombarded by a stream of electrons to convert the molecules into ions. So called propelling or accelerating electrodes propel the ions from an ionization chamber (forming part of the ion source) into and through an analyzer chamber. During passage through the analyzer chamber the ions are subjected to a transverse electric or magnetic field or both, to separate them according to their mass-tocharge ratios into a plurality of diverging beams of ions. Each ion beam is composed of ions of the same specific mass and differing from the ions in the other beam. The divering beams are successively focused on and discharged at an ion collector. The current produced by the discharge of each beam on the ion collector is a measure of the partial pressure of the molecules (from which the ions were derived) in the sample being analyzed.

The foregoing description is of one conventional type of mass spectrometer. Other types of mass spectrometers have been developed. In one such instrument the ions are caused to pursue a spiral path between an ion source and a collector. In another, the ions are caused to oscillate in a resonant field, and in yet another the ions are caused to travel in mass bunches between an ion source and an ioncollector. In any of these instruments, means are provided to ionize the molecules of the sample being analyzed. These means are commonly referred to as the ion source.

The ion source generally includes a sample .in-

let opening into an ionization chamber, a source of electrons for ionizing the sample in the ionization chamber and one or more electrodes for propelling the ions from the ionization chamber. The present invention is directed toxim provements in the electron source and contemplates application of such improvements in any ion source as used in a mass spectrometer or in other instruments wherein ionization is accomplished by electron bombardment. In-particular, the invention deals with means for mounting determination of its relative location and prevents migration or shifting after it is mounted to the ion source.

In accordance with the invention, I provide in an ion source, comprising an ionization chamher and a source of electrons, the improvement which comprises a receptacle or recess in a wall of the ion source, mounting means for the electron source including a positioning member adapted to fit within the recess, and means for afiixing the member in the receptacle.

By proper location of the receptacle With respect 'to the ion source and proper location of the positioning member with respect to the electron source, the correct location of the electron source with respect to the ionization chamber is assured. It is a big advantage in reassembling the ion source after cleaning, repairing, etc. to be assured that the electron source will be correctly located without repeated adjustment.

In a preferred embodiment of the invention as hereinafter more fully described thereceptacle and positioning member are so formed that the location of the electron source with respect to the ionization chamber is determined. In this embodiment thereceptacle is in a form of an elongated V notch formed on a wall of the ion source and running parallel to the longitudinal axis of the ion source. The upper end of the V notch is partially blocked by an overhanging shoulder. In this embodiment, the positioning member is likewise V-shaped in cross section and v is of substantially the same dimensions as the V notch in the ion source. By positioning the V-shaped positioning member in the notch so that the end of the member butts against the overhanging shoulder, movement of the electron source is prevented either laterally or parallel to the longitudinal axis of the ion source in the direction of the shoulder. By securely affixing the positioning member in the receptacle, movement away from the longitudinal axis of the ion source and movement away from the overhanging shoulder is also prevented.

The invention will be more clearly understood with reference to the following detailed description thereof which is taken in relation to the accompanying drawing in which:

Fig. 1 is a vertical section through an ion source of recent design showing the electron source mounted thereto;

Fig. 2 is a cross section taken on the line 2-2 of Fig 1;

' .Fig. 3 is an elevation of one embodiment of the the electron source which provides accurate pre-. '55 invention; and

Fig. 4 is an isometric view of another embodiment of the invention.

The ion source in Figs. 1 and 2 of the drawings is of comparative recent design and is described in detail in a copending application Serial No. 33,596, filed on June 17, 1948, now Patent No. 2,548,859, dated April 17, 1951, by Clifford E. Berry. This ion source was chosen as a means of illustrating the present invention but it is not intended to limit the invention in its application to any particular ion source. As described above, any ion source will comprise a substantially enclosed chamber, commonly referred to as an ionization chamber, having an inlet and an outlet and at least one electrode for discharging ions from the chamber after they have been formed therein, the electron source of the invention is adapted to be employed with any ion source of thisnature.

Referring now to Figs. 1,2 and 3 it will be seen that the ion source I comprises a chamber ll having an inlet 12. A pair of propelling or pusher electrodes l4, MA are disposed in the inlet end of the chamber H which is provided with an exit slit l5 opposite the pusher electrodes. Additional electrodes l6, ll, known as accelerating electrodes are frequently employed to produce acceleration of the ions issuing from the chamber H through the slit I5. The means of mounting the various electrodes with respect to the ionization chamber and means of applying difiering potentials thereto is described in detail in the aforesaid copending application and forms no part of the present invention. As shown in Fig. 2 the body of the ion source H3 is generally cylindrical in section. A portion of one wall of the source is cut away to form a fiat face extending from the lower end of the ion source (Fig. l) to a point substantially in lateral alignment with the outlet end of the ionization chamber 1 I. An arcuate shield forms with face 29, an enclosure 2| open at its lower end and closed at its upper end by an overhanging lip 23.

The electron source itself comprises a filament or wire '25 attached at opposite ends to terminal posts 21, 28. The terminal posts are sealed through an insulating base 39 and project through the opposite side of the base for connection to suitable electric leads (not shown). A post 32 is sealed in the base 39 intermediate the terminal posts 27, 28, and supports at its upper end (see Fig. 3) a positioning member 34. The positioning member is generally square in section having one of its diagonals parallel to the plane intersecting the terminal posts 21, 28. The opposite corners of the square positioning member (square when viewed in section) are flattened at least in part for reasons hereinafter made apparent.

A notch 35 is formed centrally on the flat face 20 of the ion source body, the notch being generally V-shaped in plan and running parallel to the longitudinal axis of the ion source. The positioning member has a hole 38 through it on the diagonal axis perpendicular to the plane intercepting the terminal posts 2'1, 28. The body of the ion source it is drilled and tapped at 39 through the apex of the notch 36. The upper end of the notch 38 is partially blocked by an overhanging shoulder 58 against which the upper face of the positioning member 38 is abutted when mounted to the ion source body. A screw 42 passing through the hole 38 in the positioning member and threading into the hole 39 in the ion source body holds the electron source in fixed position adjacent a slit 44 in the ion source opening into the ionization chamber H. A diametrically opposite slit 45 opens into a cavity 46 in which is disposed an electron catcher 48. As shown in Figs. 1 and 2 the post 32 upon which the positioning member is mounted is displaced forwardly from the plane intercepting the terminal posts 21, 28. This permits insertion of the positioning member in the receptacle formed by the notch 36 and proper spacing of the electron emitting filament 26 with respect to its lateral displacement from the wall of the ion source.

Assembly of the electron source in the ion source is very simple. The positioning member 38 is inserted in the receptacle formed by the notch 36 and is abutted against the shoulder 40 so as to aifix its position with respect to lateral or vertical (referring to Fig. 1) displacement. The screw 42 fixes this position of the positioning member in the receptacle and will insure the same position upon each reassembly of the apparatus.

Another embodiment of the invention is shown isometrically in Fig. 4. This embodiment is similar to that heretofore described as having a base 50 through which a pair of terminal posts 5!, 52 are sealed and a filament 54 afiixed at opposite ends to the terminal posts 5!, 52. Also a positioning member 58 is supported above the base 59 by a post 5? sealed in the base intermediate the terminal posts 5|, 52. The shape of the positioning member 55 is substantially similar to that of positioning member 38 shown in Fig. 3 and the means of mounting the electron source of Fig. 4 to the ion source is identical to that described with relation to the embodiment of Fig. 3.

The electron source of Fig. 4 differs from that heretofore described in having a pair of auxiliary conductive posts 68, El, sealed through the base 50 and extending parallel to the terminal posts 51, 52 and just inside of the terminal posts. A semi-circular conductive shield 64 is mounted to the upper end of the terminal posts Bil, 6| so as to extend part way around the filament 59. By application of a potential to the shield 64 through the terminal posts 60, Gl the electrons emitted from the filament tend to be focussed in a plane passing through the focal point of the shield. If the electron source of Fig. 4 were mounted in the manner in which the electron source 26 is mounted in the apparatus of Fig. l, the shield as would open toward the slit 44 and would serve to focus a higher percentage of the electrons emitted from the filament through the slit 4%. By using a focussing shield or electrode, the dimensions of the slit become less important. An electron beam is formed by the shield rather than by the slit.

The use of a filament of circular cross section together with a focussing electrode or shield is described by Harold W. Washburn in United States patent application Serial No. 52,341, filed October 1, 1948. The use of such a filament and focussing electrode forms no part of the present invention apart from the particular means shown for mounting the same.

I claim:

1. In an ion source comprising an ionization chamber and a source of electrons, the improvement which comprises a recess in a wall of the ion source, mounting means for the electron source, said mounting means including a member positioned and shaped to fit within said recess to .fix the. location of said mounting means with respect to the ion source, and means for affixing said member in said recess.

2. In an ion source comprising an ionization chamber and a source of electrons, the improvement which comprises a slit in a wail of the source opening into the ionization chamber, a recess in a wall of the ion source, mounting means for mounting the electron source adjacent the slit, said mounting means including a member positioned and shaped to fit within said recess to fix the location of said electron source with respect to the slit, and means for affixing said member in said recess.

3. In an ion source comprising an ionization chamber and a source of electrons, the improvement which comprises a slit in a wall of the ion source opening into the ionization chamber, a recess in a wall of the ion source, mounting means for the electron source, said mounting means including a member positioned and shaped to fit within said recess to fix the location of said electron source with respect to the slit, and means for afiixing said positioning memher in said receptacle.

4. Apparatus according to claim 3 wherein the electron source comprises a filament mounted at opposite ends to a pair of terminal posts.

5. In an ion source comprising an ionization chamber and a source of electrons, the improvement which comprises a slit in a wall of the ion source opening into the ionization chamher, a recess in a wall of the ion source, mounting means for the electron source, said mounting means including a base, a pair of spaced terminal posts extending through the base and amxed thereto, a filament affixed at opposite ends to the terminal posts, a member positioned and shaped to fit within said recess to fix the location of the filament with respect to the slit, and means for securing said positioning member in the recess.

6. In an ion source comprising an ionization chamber and a source of electrons, the improvement which comprises a slit in a wall of the ion source opening into the ionization chamber, a recess in a wall of the ion source, mounting means for the electron source, said mounting means including a base, a pair of spaced terminal posts extending through the base and affixed thereto, a filament afiixed at opposite ends to the terminal posts, a support post extending from the base between the terminal posts, a member afiixed to the support post and positioned and shaped to fit within said recess to fix the location of the filament with respect to the slit, and means for securing said positioning member in the recess.

7. Apparatus according to claim 6 wherein the base of the mounting means is glass.

8. In an ion source comprising an ionization chamber and. an electron source, the improvement which comprises a slit in a wall of the ion source gpening into the ionization chamber, an elongated notch in a wall of the ion source, mounting means for the electron source, said mounting means including a base, a pair of spaced terminal posts extending through the base and afiixed thereto, a filament affixed at opposite ends to the ends of the terminal posts, a support post extending from the base between the terminal posts, a positioning member affixed to the support post and adapted to fit Within said notch to fix the location of the filament with respect to the slit, and means for securing said positioning member in the notch.

9. In an ion source comprising an ionization chamber and an electron source, the improvement which comprises a slit in a wall of the ion source opening into the ionization chamber, an elongated notch in a wall of the ion source extending parallel to the longitudinal axis of the ion source and being open at its lower end and at least partially blocked at its upper end, mounting means for the electron source, said mounting means including a base, a pair of spaced terminal posts extending through the base and aflixed thereto, a filament afiixed at opposite ends to the ends of the terminal posts, a support post extending from the base between the terminal posts, an elongated positioning member affixed to the support post and adapted to fit within said notch to fix the location of the filament with respect to the slit, and means for securing said positioning member in the notch.

10. Apparatus according to claim 9 wherein the notch is V shaped and opens outwardly and the positioning member is substantially square when viewed in horizontal section and is mounted on said mounting post with a diagonal plane thereof parallel to the plane intersecting the two terminal posts so that a portion of the member will fit snugly in the notch and will abutt against the blocked end thereof.

11. In an ion source comprising an ionization chamber and a source of electrons, the improvement which comprises a slit in a wall of the ion.

source opening into the ionization chamber, a recess in a wall of the ion source, mounting means for the electron source, said mounting means including a base, a first pair of spaced terminal posts extending through the base and aflixed thereto, a filament afiixed at opposite ends to the terminal posts, a second pair of spaced terminal posts mounted through the base inside the first pair of posts and parallel thereto, a semi-cylindrical metal shield mounted between the second pair of spaced posts with the filament substantially co-axial therewith, a support post extending from the base between the second pair of terminal posts a positioning member aflixed to the support post and adapted to fit within said recess to fix the location of the filament with respect to the slit, and means for securing said positioning member in the recess.

GORDON W. PALLE'I'IE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 711,622 Fergusson Oct. 21, 1902 948,813 Hilton et al. Feb. 21, 1911 2,444,968 Washburn July 13, 1948 2,488,196 Janssen Nov. 15, 1949 OTHER REFERENCES Coggeshall and Jordan, Review of Scientific Instruments, May, 1943, vol. 14, pp. -129. 

